1. Field of the Invention
The present invention relates to a cordierite sintered body, a method for manufacturing the same, a composite substrate, and an electronic device.
2. Description of the Related Art
Since being a material having a high heat resistance and a low coefficient of thermal expansion, a cordierite sintered body has been known as a material having a high heat shock resistance. In particular, a porously sintered structural body has been widely used as a filter or a catalyst carrier for cleaning of exhaust gas emitted from automobiles and the like.
In recent years, by the use of characteristics, such as a low coefficient of thermal expansion and light weight, a dense cordierite sintered body has been increasingly employed as a stage member (Patent Literature 1) of an exposure apparatus or the like or an ultra-precise mirror substrate (Patent Literature 2). According to Patent Literature 1, in particular, in order to obtain a high rigidity, improvement has been made in such a way that a CaO content of the cordierite sintered body is set to 0.2 to 0.8 percent by mass, and as a subcrystal, a predetermined amount of Al2O3 is contained. It has been believed that CaO has an effect of improving Young's modulus of cordierite by promoting grain growth and sintering properties thereof and that Al2O3 has an effect of densifying cordierite by suppressing abnormal grain growth thereof. According to Patent Literature 2, besides obtaining a high rigidity, in order to decrease the surface roughness, a predetermined amount of a specific rare earth metal component is added as a sintering auxiliary agent, so that a dense cordierite sintered body is formed. In the sintered body thus obtained, crystal phases other than that of cordierite are not contained, and the rare earth metal-containing component is present in the form of a membrane along the grain boundary of cordierite grains as an amorphous phase. It has been believed that since the crystal components other than that of cordierite are not present, the generation of surface irregularities due to the difference in polishing properties between different grains can be avoided.
An example in which a dense cordierite sintered body is formed without adding a sintering auxiliary agent has been disclosed in Patent Literature 3. In this example, a molded body obtained by uniaxial die press molding of a cordierite powder having an average grain diameter of 0.7 μm or less is fired at 1,400° C. for 12 hours in a nitrogen atmosphere, so that a cordierite sintered body is formed which has characteristics, such as a cordierite content of 97.6 percent by mass, a bulk density of 2.54 g/cm3, an open porosity of 0%, and a total porosity of 0.1%, and which has different phases of mullite, spinel, and sapphirine (Example 1). It is found that this sintered body has a closed porosity of 0.1% from the total porosity and the open porosity and that from a photo of a polished surface after thermal etching shown in FIG. 2, approximately 20 closed pores having a major axis of approximately 0.2 to 0.5 μm are present in a surface having an area of approximately 20 μm2.
In addition, in recent years, as a surface acoustic wave device, the structure in which a main substrate and an auxiliary substrate are joined to each other has been developed. For example, Patent Literature 4 has disclosed a surface acoustic wave device in which a main substrate formed of lithium tantalate, lithium niobate, or the like and an auxiliary substrate formed of glass or silicon are directly joined to each other. In this surface acoustic wave device, the coefficient of thermal expansion of the auxiliary substrate is lower than that of the main substrate, and the thickness of the auxiliary substrate is larger than that or the main substrate. When the main substrate and the auxiliary substrate as described above are used in combination, in the case in which the temperature of the substrate is increased, a compressive stress works in the vicinity of the surface of the main substrate, and a smaller thermal expansion than the inherent thermal expansion of the main substrate is obtained. As a result, it has been explained that the frequency temperature dependence of the surface acoustic wave device using the main substrate is improved. In addition, it has also been explained that in the case in which the auxiliary substance is formed of glass, since the coefficient of thermal expansion thereof is 4.5 ppm/° C., and the glass has amorphous properties, joining with the main substrate which is a single crystal can be easily performed. However, the surface condition of the main substrate and that of the auxiliary substrate, which are to be joined to each other, have not been described in detail.
Patent Literature 5 has disclosed, as is the case of Patent Literature 4, a technique of improving the temperature dependence of a surface acoustic wave device. A piezoelectric substrate (main substrate) is formed of lithium tantalate or lithium niobate, a support substrate (auxiliary substrate) is formed of sapphire, aluminum oxide, aluminum nitride, or silicon nitride (coefficient of thermal expansion: 2.6 ppm/° C.), and a joined substrate is formed by direct joining. However, the surface condition required for the support substrate and tie like has not been described.